1. Field of the Invention
The present invention relates to a controller for a motor which has two rotors individually producing fields by permanent magnets and which allows a phase difference between the two rotors to be changed.
2. Description of the Related Art
In a permanent-magnet type motor, there has conventionally been known a motor of a double-rotor structure in which each of two rotors coaxially disposed has permanent magnets that produce fields (refer to, for example, Japanese Patent Application Publication No. 2002-204541 (hereinafter referred to as patent document 1)). In this type of motor, the two rotors can be relatively rotated about their axes. The relative rotation allows a phase difference between the two rotors to be changed. Further, changing the phase difference between the two rotors makes it possible to change the intensity of a resultant field (the magnitude of a magnetic flux) obtained by combining the fields produced by the permanent magnets of the rotors.
The motor disclosed in the aforesaid patent document 1 is constructed such that the phase difference between the two rotors mechanically changes according to the rotational velocity of the motor. More specifically, the two rotors are connected through the intermediary of a member that is displaced in the radial direction of the motor when subjected to a centrifugal force. One of the two rotors is configured such that it can rotate integrally with an output shaft that outputs generated torque of the motor to an external source. Further, the other rotor is configured such that, as the aforesaid member is displaced, the other rotor relatively rotates with respect to the other rotor that can rotate integrally with the output shaft, thereby changing the phase difference between the two rotors. In this case, the permanent magnets of the rotors are arranged such that, when the motor is at a halt, the magnetic poles, i.e., the magnetic fluxes, of the permanent magnets provided in the two rotors are oriented in the same direction, causing a resultant field of the permanent magnets to provide a highest intensity. As the rotational velocity of the motor increases, the phase difference between the two rotors changes due to a centrifugal force and the intensity of the resultant field of the permanent magnets of the two rotors decreases.
As described above, in the motor wherein the intensity of a resultant field of the permanent magnets of the two rotors can be changed, an expanded operating range of the motor and higher energy efficiency of the motor can be effectively achieved by properly changing the resultant field.
However, in the motor disclosed in the aforesaid patent document 1, the phase difference between the rotors is simply mechanically changed according to the rotational velocity of the output shaft, making it difficult to accomplish detailed control. Therefore, it has been difficult to effectively expand the operating range of the motor or to effectively improve the energy efficiency of the motor.
Hence, the present applicant has made an attempt to actively control the phase difference between the rotors by using, for example, a hydraulic device. For instance, a hydraulic chamber whose volume changes as a second rotor relatively rotates is created beforehand, and the second rotor is rotated relatively to a first rotor by the pressure of hydraulic oil charged in the hydraulic chamber.
Using the hydraulic device makes it possible to control the phase difference between the rotors to a desired phase difference by adjusting the pressure in the hydraulic chamber. Consequently, the intensity of a resultant field of the two rotors can be controlled to a desired intensity.
In general, however, the phase difference between the two rotors need not be frequently changed, and it is maintained to be constant in many cases. For this reason, sludge tends to build up in the hydraulic chamber. Such accumulated sludge may cause a failure of the mechanism for changing the phase difference between the rotors. An example of the failure is unsmooth rotation of the other rotor of the two rotors relative to the rotor that can be integrally rotated with the output shaft. Hence, there has been a demand for measures for removing the sludge, as necessary.